The present invention relates to an electronic device formed by fixing a substrate, on which a heat generating component and a non-heat-generating component are mounted, to a heat radiating body.
An electronic device having a substrate secured to a heat radiating body such as an aluminum block or heat sink is known. A heat generating component and a non-heat-generating component are mounted on the substrate. A thus configured electronic device efficiently radiates heat from the heat generating component. For example, a heat generating component mounted on a substrate used in a vehicle generates a relatively great amount of heat. In this case, a metal substrate exhibiting improved heat radiating performance is employed for the substrate.
FIGS. 7A and 7B show an example of an electronic device having a highly heat radiating metal substrate. Specifically, a metal substrate 51 has an aluminum core 52 and an insulating layer 53 formed of resin, which is provided on the core 52. A circuit pattern (not shown) of copper (Cu) is formed on the insulating layer 53. Heat generating components 54 and non-heat-generating components 55 are mounted on the circuit pattern. The metal substrate 51 is fixed to a seat 57 of a heat radiating body (a box-like body) 56 formed of aluminum by means of, for example, three screws 58. In such fixing, a small gap is formed between the backside of the metal substrate 51 and the seat 57. The gap is filled with a silicone grease 59, thus ensuring heat radiation from the heat generating components 54.
In an electronic device described in Japanese Laid-Open Patent Publication No. 2004-221111, referring to FIGS. 8A and 8B, a heat generating component 63 having a metal substrate 62, on which a heat generating element 61 is mounted, is fixed to a heat radiating body 65 by means of bolts 66 through an oil compound 64. A groove 67 is defined in a securing surface of the heat generating component 63 that faces the heat radiating body 65. The securing surface is divided into a plurality of sections by the groove 67. The oil compound 64 is applied onto one of these sections of the securing surface located in the proximity of a position immediately below the heat generating element 61. In correspondence with the groove 67, an optimal application range of the oil compound 64 is determined. Further, the groove 67 prevents excessive oil compound 64 from flowing to the outside of the heat generating component 63. This improves the appearance of the electronic device and facilitates application of the oil compound 64.
Recently, use of lead-free solder has been promoted in mounting of chip components (non-heat-generating components). However, compared to lead-containing eutectic solder, the lead-free solder exhibits low ductility and is fragile. Therefore, if a non-heat-generating component is mounted on a substrate using lead-free solder, soldered joint portions are easily influenced by stress.
In the device of FIGS. 7A and 7B, the backside of the metal substrate 51 and the surface of the seat 57 are not flat but uneven. Therefore, when the metal substrate 51 is simply fixed to the seat 57, the metal substrate 51 contacts the seat 57 through projecting portions. This reduces the contact area between the metal substrate 51 and the seat 57, decreasing heat conductivity. To solve this problem, the silicone grease 59 (heat conductive oil) is applied between the metal substrate 51 and the seat 57. However, if the metal substrate 51 is fixed to the seat 57 by means of the screws 58 with the silicone grease 59 arranged between the metal substrate 51 and the seat 57, the silicone grease 59 causes deformation of the metal substrate 51, applying stress to the non-heat-generating components 55 mounted on the metal substrate 51. Particularly, in a thermal shock test, it was demonstrated that a great stress act on the soldered joint portion of each of the non-heat-generating components 55. Specifically, the stress does not cause a crack in a soldered joint portion between the metal substrate 51 and the seat 57 that are fixed together using eutectic solder. However, a crack and a resulting rupture are easily generated by the stress in a soldered joint portion of the metal substrate 51 and the seat 57 that are fixed together using lead-free solder.
As described in Japanese Laid-Open Patent Publication No. 2004-221111, the groove 67 defined in the securing surface of the heat generating component 63 prevents excessive oil compound 64 from flowing to the outside of the heat generating component 63. Nonetheless, the technique does not describe or suggest a countermeasure to prevent a crack in the soldered joint portions of each non-heat-generating component mounted on the metal substrate, which is caused by the stress produced through warpage of the metal substrate.